The present invention relates generally to image forming instruments which present at or near infinity an image of a wide field of view. More particularly, the invention relates to an optical system for a visual vehicle simulator display having a large exit pupil for simultaneously accommodating both of the user's eyes and providing the user with a considerable degree of freedom for head movement before the visual corrected image is lost with one or the other of the user's eyes.
As the cost of training operators in sophisticated vehicles such as modern, commercial and military aircraft has risen, there has been an increased demand for better simulators and trainers in which training can be accomplished at a much lower cost. One requirement of many of these vehicle simulators or trainers is a visual projection system which can provide the trainee with visual cues.
In the past, image projection systems for these types of simulators have taken the form of a mirror and beam splitter for projecting the image generated by a cathode ray tube or the like. In one prior art arrangement, a cathode ray tube is disposed to the side of the central axis of the projection system with its screen extending in a direction generally parallel to the central axis of the projection system. The image from the cathode ray tube screen, located at the focus of a spherical mirror, is projected on to a tilted beam splitter which is centered on the central axis of the projection system. An image is reflected from the beam splitter to a spherical mirror which reflects the image along the central axis of the projection system back through the beam splitter toward the eye of the observer. The observer is positioned close to the center of curvature of the spherical mirror such that light which reaches the observer appears to be coming from infinity just as would occur in the real world. Such mirror beam splitter display systems are described in U.S. Pat. No. 3,432,219 and some of the references cited within. Also Pat. Nos. 3,659,920 and 3,709,581 disclose various forms of mirror beam splitter arrangements.
All prior art mirror beam splitter optical systems suffer from a common problem, namely, poor light transmission. In the case of the system previously described, only approximately twenty percent of the light generated by the cathode ray tube can be transmitted through the system. An additional problem with the optical system just described is that the side-mounted cathode ray tube and tilted beam splitter prevents the optical system from being positioned close to a vehicle on which training is being simulated. The side-mounted cathode ray tube and the tilted beam splitter also complicates the joining or abutting of several optical systems for achieving a contiguous field of view for training purposes. Abutting of optical projection systems in this manner is highly desirable when trying to design a simulator for a vehicle such as a military fighter which requires a wide field of view.
Attempts to alleviate the aforementioned problems relating to the side-mounted cathode ray tube and the tilted beam splitter have involved the use of a two way spherical mirror beam splitter. Such a system allows the screen of the cathode ray tube to be centered on the central axis of the projection system. The image generated by the cathode ray tube is then projected through the back of the spherical mirror and on to a beam splitter disposed directly in front of the spherical mirror in a perpendicular relationship to the central axis of the projection system. The image is then reflected from the beam splitter back to the spherical mirror which then reflects the image back through the beam splitter to the eyes of the observer which are disposed close to the center of curvature of the mirror. While this approach allows the screen of the cathode ray tube to be centered on the central axis of the projection system and facilitates the placement of projection systems in abutting relationship to create a large contiguous field of view, the transmissability of these types of systems is as low as one percent. As a result, cathode ray tubes having specially designed phosphor screens are necessary and it is necessary to drive the cathode ray tube very hard to create enough light for generating a usable training image. The cathode ray tubes used in these prior art systems are expensive and relatively unreliable. Furthermore, the amount of light transmitted through either types of prior art mirror beam splitter projection systems is so low so as to create an image which is of only marginal quality.